1. Field of the Invention
The invention concerns fibers and flat textile structures such as mats, for example, made of fibers, which are coated with a preparation which has the property of decomposing during thermal treatment of the flat structure. Such decomposition may result in thermal strengthening, for example, so that flat structures with improved charge stability are obtained.
2. Description of Related Technology
Flat textile structures with an applied electrostatic charge are suitable, for example, for applications in filter technology because, in comparison to uncharged flat structures, they may have enhanced filtering action. Those flat structures which contain at least partly electret fibers are especially suitable for such flat structures. Electret fibers in the sense of the invention are fibers made of electrically nonconducting materials, which are able to store an applied electrostatic charge over a long time period (up to several years). Mats with increased charge stability are described in EP-A-0 705 931 (Apr. 10, 1996) and counterpart U.S. Pat. No. 5,726,107 (Mar. 10, 1998), the entire disclosures of which are incorporated herein by reference. According to these publications, charge stability can be improved by adding a charge control agent to the fiber-forming materials (polymers and polycondensates) in the electret fibers, and when different materials are used for the fiber-forming material. The mats of the prior art that can be used as filter material consist essentially of different types of fibers, but not all fibers present are necessarily electret fibers, that is, other fiber types may be present in certain amounts, depending on the desired profile of the properties.
If the fibers required to produce the filter are produced by the usual spinning methods then, during manufacture, they must be provided with an antistatic preparation because, otherwise, the frictional processes that occur during the production of fibers and especially during the production of flat textile structures lead to charging and thus to uncontrollable forces acting on the fibers. According to EP-A-0 705 931 and U.S. Pat. No. 5,726,107, a water-soluble antistatic preparation is applied onto the fibers during manufacture and is then washed out after the production of the mat. The electrostatic charging of the filter is done finally either by a special charging step, for example, a corona discharge, or quasi in situ during the operation of the filters via the triboelectric effect that occurs as a result of friction between the medium to be filtered and the filter material. The disadvantage of the method described is that the preparations mentioned above can be removed from the mat only by washing to a sufficient extent. If a washing process is carried out on an industrial scale, this may lead to irreversible mechanical damage of the mat, which would then have an adverse effect on its use as filter.
Other preparations for the production of staple fiber materials are described in EP-A-0 265 221 (Apr. 27, 1988) and counterpart U.S. Pat. No. 4,818,599 (Apr. 4, 1989). These preparations are vulcanized polysiloxanes or hydrophilic coatings containing polyalkylene oxide chains or segments, for example, a block copolymer of polyethylene oxide and polyethylene terephthalate chain members. These compounds are vulcanized at about 170.degree. C. and after processing the fibers to flat structures, can no longer be removed by thermal treatment without damaging the fibers.
The preparations which are known in the art have the disadvantage that, after the formation of the flat textile structure, they can no longer be removed in a simple manner by thermal treatment without residues. Now it was found that the filter action can be improved and yet satisfactory properties are obtained during the fiber manufacture.